The following disclosure is based on German Patent Application No. 102 25 842.2 filed on Jun. 4, 2002, which is incorporated into this application by reference.
1. Field of the Invention
The invention relates to a method for determining the radiation-damage resistance of an optical material. More particularly, the invention relates to such a method that involves simultaneously irradiating several sample volumes of the optical material with test radiation from a common radiation source having differing, measured or preset, radiant-energy densities and measuring at least one parameter indicative of the radiation-damage resistances of the irradiated sample volumes using measuring radiation. The invention further relates to a device suitable for carrying out such a method.
2. Description of the Related Art
It is known that the optical quality of optical materials, i.e., materials, such as calcium fluoride or synthetic quartz glass, that serve a function in optical components, degrade over their service lives due to the aggregate radiation doses they have received. For example, the material""s transmittance decreases over its service life due to the radiation dose it has received, a phenomenon that, in particular, also occurs in the case of applications involving ultraviolet laser radiation.
There is thus need for methods and devices that will allow determining the resistances of optical materials to damage due to irradiation to which they are subjected while in use in order to, e.g., allow predicting their service lives. A known method for obtaining meaningful results within reasonable measurement periods, which are orders of magnitude shorter than the typical service lives of optical materials, involves irradiating samples of the optical material involved employing radiant-energy densities that are significantly greater than those typically occurring in normal use. The results of measuring one or more parameters indicative of its resistance to radiation damage obtained over a range of such high radiant-energy densities are then extrapolated to the range of radiant-energy densities applicable to normal use in order to allow making statements regarding the radiation-damage resistance of the material in normal use and thus, e.g., regarding its maximum service life.
That extrapolation requires making several measurements employing various, high, radiant-energy-densities. The greater the total number of such measurements that are available and the more the radiant-energy levels employed differ from one another, the more reliably and meaningfully the measurement results obtained may be extrapolated to the interesting range of radiant-energy densities that typically occur in normal use. In that conjunction, a known method involves making several measurements on one or more samples of the optical material at various radiant-energy densities, where each of the various radiant-energy densities employed on a given sample is provided by its own, individual, radiation source, or by a single radiation source with an adjustable output that irradiates the samples consecutively, one after the other, with the various radiant-energy densities.
U.S. Pat. No. 6,075,607 describes a method and a device of that type for carrying out that method for model-based determination of the resistances of optical materials to damage by pulsed excimer-laser radiation, in which measurements of the absorption coefficients or transmittances, as parameters indicative of resistance to radiation damage, of a sample irradiated by differing energy densities are recorded over both a range in which linear functional dependence applies and a saturation region that corresponds to greater energy densities in order to then derive and correlate approximation equations yielding the functional dependence of those absorption coefficients or transmittances on irradiation energy density or the number of laser pulses using statistical and theoretical methods. The samples to be measured are preferably obtained by cleaving a large block.
Other methods and devices for determining the resistance of optical materials to radiation damage that involve conducting repeated measurements on a single sample, or various samples, using a test beam having the desired radiant-energy density for each of the measurements are disclosed in patent applications EP 0 905 505 A1, JP 2001-099753 A, JP 2000-099751 A, JP 2000-180301 A, JP 11-230859 A, JP 11-118669 A, JP 10-232184 A, and JP 10-232197 A.
Patent application JP 11-258108 A describes a method and a device for determining the resistance of an optical material to damage by laser radiation, in which a sample is repeatedly irradiated by a laser beam at differing radiant-energy densities and the absorbed portion is determined using a piezoelectric sensor each time it is irradiated, where the sample consists of a substrate and an optical coating in the form of an antireflective film or a reflective coating. In addition, a portion of the irradiation that is transmitted or reflected by this sample is directed to another sample consisting of the uncoated substrate material only. One additional sample can also be irradiated with the reflected and transmitted light, respectively. A lens may be arranged in front of each additional sample. The absorbed portion(s) of the radiation are also measured for one or both of these other samples using a piezoelectric sensor. The measurement results recorded for one or both of these other samples are then correlated to the measured values obtained for the coated sample in order to improve the accuracy of the relation governing the behavior of the latter.
The magazine article, C. K. Van Peski, et al: xe2x80x9cBehaviour of Fused Silica Irradiated by Low Level 193 nm Excimer Laser for Tens of Billions of Pulses,xe2x80x9d Proc. SPIE, Vol. 4347, p. 177 (2001), presents the results of investigations of the behavior of synthetic quartz glass under irradiation by several tens of billions of pulses of excimer-laser radiation at a wavelength of 193 nm at low energy densities over extended time periods. For the purposes of those investigations, six samples of the quartz-glass material were lined up, one behind the other, on an associated test setup. The UV laser beam emitted by an ArF-laser was initially guided through the six samples lined up one behind the other as a first beam passing through a first volumnar zone, then deflected and guided back through the six samples in the reverse order as a second beam passing through a second volumnar zone, then redeflected and once again guided through the six samples as a third beam passing through a third volumnar zone, and, finally, redeflected again and guided back through the six samples as a fourth beam passing through a fourth volumnar zone, yielding a total of 24 sampled volumes, where the first sample volume irradiated by the first beam is irradiated by an energy density of 0.2 mJ/cm2 and the other sampled volumes were successively irradiated with stepwise decreasing energy density. The investigation was done over a time period of 133 days, respectively interrupted for measurement procedures. During those measurement procedures, the effects of the irradiation on the material were investigated employing three different methods, firstly, interferometric measurements of wavefront distortions for transmitted radiation, secondly, birefringence measurements at a wavelength of 632 nm, and, thirdly, FTIR spectral analysis.
It is an object of the invention to provide a method and a device of the type mentioned at the outset hereof with which the resistance of an optical material to damage by radiation to which it is subjected in use may be comparatively reliably determined with relatively simple instrumentation in a relatively short time.
The invention achieves this and further objects by providing a method and a device for determining the resistance of an optical material to radiation damage having the characteristics that the measuring radiation comes from the same radiation source as the test radiation and the optical material""s resistance to radiation damage is determined based on a functional relation between the damage-resistance parameter(s) and the radiant energy densities, which is determined from the values of the damage-resistance parameter(s) measured for the various sample volumes at the various radiant-energy densities.
In the case of the method according to the invention and the device according to the invention, several sample volumes of the optical material are simultaneously irradiated with test radiation that comes from the same radiation source, where the sample volumes are irradiated with test radiation having differing radiant-energy densities. One or more parameters, such as transmittance and/or absorptance, which are indicative of their resistances to radiation damage, are measured at the sample volumes using measuring radiation, which also comes from the one radiation source, and correlated to the radiant-energy density for the particular sample volume involved. This then allows determining the resistance of the optical material to radiation damage, based on a functional relation between that parameter and radiant-energy densities, which is determined using the measured values of the radiation-damage-resistance parameter of the various sample volumes for the various radiant-energy densities involved.
The invention thus allows very rapidly obtaining measured values of parameters representative of resistance to radiation damage at various radiant-energy densities using relatively simple instrumentation, in particular, using just a single radiation source, from which reliable statements regarding the resistance of the optical material to radiation damage and thus also regarding, e.g., its expected service life in normal use, where radiant energy densities that are much less than that of the test radiation employed are usually employed, after a relatively short time, particularly if high energy densities are employed. When making such service-life estimates, the functional relation that expresses one or more radiation-damage-resistance parameters as a function of radiant-energy density that was determined during testing, preferably for a range of high energy densities, may be extrapolated to the range of energy densities that apply during normal use of the optical material. That extrapolation is preferably performed with the aid of a model.
Under another embodiment of the invention, the measuring radiation is coupled out of the respective sample volume involved in the form of a portion of the test radiation, i.e., the measurement is performed simultaneously with the irradiation by making use of the test radiation.
The sample volumes may be portions of a single test sample, i.e., test radiation passes through the test sample and the one or more radiation-damage-resistance parameters are then measured at several partial volumes of the test sample that follow one another in the beam path.
Alternatively, or additionally, sequential arrangements of sample volumes of separate test samples sequentially arrayed, one after the other, along the beam path traversed by the test radiation may be formed. Under a beneficial configuration of this measure, a variable attenuator, with which the radiant-energy density of the test radiation may be variably attenuated in a controlled manner, is arranged between each pair of sequentially arrayed test samples. Employment of such attenuators will allow maintaining the energy densities at the various sample volumes substantially constant at their differing, initial, values over the full duration of testing, even though test-radiation energy-density losses at the sample volumes will, in general, decrease over that period of time due to radiation-induced aging of the material involved, which may be compensated by setting the attenuators to greater attenuation factors at the start of testing and then resetting them to lesser attenuation factors as testing progresses.
Under another embodiment of the invention, transmittance is employed as a radiation-damage-resistance parameter. The transmittances of the respective sample volumes are either determined from the transmitted fractions of the test radiation or measured using a measuring beam directed at the sample volumes.